This invention relates in general to signal processing with reference to a composite video signal, and more particularly to the extraction of luminance information from a video signal on which color information has been encoded.
A broad range of signal processing is frequently performed on video signals upon which color information has been encoded. This includes not only the storing and subsequent retrieval of the video information from a wide variety of media, including magnetic tape, disk, and semiconductor memory devices, but also real time algorithmic type processing including color correction and noise reduction. In connection with such signal processing, it is freqeuntly desirable to reduce the video information contained in the composite video signal into its respective component forms, and thereafter perform the desired processing upon the respective components.
Basically speaking, a television picture is composed of a plurality of horizontal lines, each line containing color and brightness information for the respective portions along its length. The total number of said horizontal lines are further subdivided into two groups, or fields in such a manner that the respective lines of each field vertically interlace to form a single picture, or frame. The signal information for each location along each of said horizontal line is encoded in a single electronic signal commonly referred to as a composite video signal. The composite video signal contains both color as well as black and white information. The average DC level of said composite signal with reference to a horizontal line generally represents the corresponding black and white brightness information. Color information is contained on a 3.58 MHz subcarrier signal which is superimposed on said DC level. In particular, the amplitude of said subcarrier contains color saturation information, and the phase angle of said subcarrier with reference to a reference subcarrier contains the respective color information. As color information is phase encoded on the 3.58 MHz subcarrier, a 3.58 MHz reference signal is transmitted with each line of video information for reference purposes. The DC level of said video signal is generally referred to as containing the luminance information, and the 3.58 MHz subcarrier is generally referred to as containing the chrominance information.
From a mathematical standpoint, the encoded color information present in the chrominance signal can be represented as a vector having a magnitude proportional to the amplitude of said subcarrier and a phase angle proportional to the phase of said 3.58 MHz with reference to said reference signal.
In considering a composite video signal with reference to the frequency domain, the luminance signal would be composed of frequency components up to approximately 4.5 MHz. As chrominance information is contained on the 3.58 MHz subcarrier, it is clear that most chrominance frequency information will be centered about 3.58 MHz. From the foregoing it is apparent that chrominance information is present within the same frequency spectrum as luminance information.
In the processing of video signals, it is frequently desirable to extract the luminance and chrominance signal components from the composite video signal. Thereafter signal processing can be performed upon the luminance and chrominance signal components directly, from which the composite video signal can be subsequently reconstructed.
In dealing with the chrominance information, while said information is present in the amplitude and phase relations of the 3.58 MHz subcarrier, it is generally not convenient to perform the desired processing upon the chominance information in this form. Rather it is preferable to obtain chrominance information in its respective component form, i.e., the R - Y and B - Y components
Several techniques are commonly used to extract chominance and luminance information from a composite video signal. In a first approach, a composite video signal is supplied to a bandpass filter having a center frequency of 3.58 MHz. The output from said bandpass filter will consequently contain primarily chrominance information. Luminance information can be thereafter derived by subtracting the output of the bandpass filter, i.e., the chrominance, from the composite video signal, thereby yielding the luminance signal. The respective R - Y and B - Y components are thereafter derived from the chrominance signal by a demodulation process. In particular, as the R - Y and B - Y signals represent the orthogonal components of the chrominance signal, said components are generally derived by multiplying the chrominance signal by a second signal. The R - Y component is generally derived by multiplying the said chrominance signal by a first reference signal having a frequency of 3.58 MHz. The resultant signal from said multiplication process is thereafter supplied to a low pass filter the output of which represents the corresponding R - Y orthogonal component in an analog format. In a similar fashion, the B - Y orthogonal component of chrominance is derived by multiplying the chrominance signal by a second reference signal having a frequency of 3.58 MHz and a phase difference from said first reference signal of 90.degree.. Thereafter the result of said second multiplication process is supplied to a low pass filter, the output of which is an analog signal representative of the B - Y orthogonal component of the chrominance signal.
While the above approach will produce a luminance signal as well as the respective orthogonal components of the chrominance signal, there are a number of short-comings. In particular, due to the fact that the luminance signal can have components in the frequency spectrum occupied by the chrominance signal, the bandpass filter employed in the above approach has limited abilities to effectively separate the luminance signal from the chrominance signal. Consequently luminance signal components are frequently present in the R - Y and B - Y output from the demodulation process.
An improved approach over the single bandpass system above described employs the operation of a comb filter. In particular the output from the bandpass filter is thereafter supplied as an input to a comb filter. The output from the comb filter represents the desired chrominance signal which is subsequently used for subtraction from the composite signal to generate a luminance signal, as well as the chrominance input for the demodulation process.
A comb filter may be implemented by taking advantage of certain phase relationships present in the 3.58 MHz subcarrier between subsequent lines, fields and frames in a composite video signal generated in accordance with the National Television System Committee standard RS-170A as promulgated by the Electronic Industries Association, or the Phase Alternating Line standard, hereinafter referred to as NTSC and PAL respectively.
In a composite video signal, the phase relation of 3.58 MHz subcarrier will change between subsequent lines in a field, as well as adjacent lines in adjacent fields. In particular there is a 180.degree. phase shift of the 3.58 MHz subcarrier between corresponding points on adjacent lines within the same field in a composite video signal generated according to the NTSC standard, and a 90.degree. phase shift between corresponding points on adjacent lines within the same field in a composite video signal generated according to the PAL standard. It is also observed that a 180.degree. phase shift exists between corresponding points on certain adjacent lines between two fields in a video signal produced according to both the NTSC and PAL standard. This phase shift can be used to an advantage in the extraction of chrominance information from a composite video signal.
In the foregoing approach employing a bandpass filter to produce a chrominance signal, by supplying the output from said bandpass filter to a delay means capable of delaying said chrominance signal by an amount equal in time to one horizontal line in the NTSC system, or two horizontal lines in the PAL system and thereafter subtracting the results from the output from the bandpass filter, the result of said process will be a signal from which luminance components have been removed and chromiance components present therein will have been doubled. This signal can then be used as an input to the demodulator for subsequent production of the R - Y and B - Y orthogonal components of the chrominance signal, as well as subtracted from the composite video signal produce a luminance signal.
While this approach can be used to produce performance superior to that achievable with the use of a bandpass filter alone, a number of shortcomings nevertheless remain. These shortcomings generally relate to the point in the video signal at which said 180.degree. phase shift occurs. In the NTSC system, the point at which a 180.degree. phase shift has occurred is available at the corresponding point in either the subsequent line in the same field, or the corresponding point on an adjacent line in the next succeeding field. In this regard it will be particularly observed that the amount of change in luminance between subsequent lines in the same field and adjacent lines in a subsequent field can vary considerably. This problem is further compounded in the PAL system wherein the phase shift between corresponding points on subsequent lines in a single field is 90.degree.. Consequently the amount of delay necessary for the video signal in the PAL system required for a 180.degree. phase reversal is two horizontal lines within a field. Clearly the amount of change in luminance possible between two horizontal lines in the PAL system further compounds the problem. In the NTSC and PAL systems, however, there is a 180.degree. phase shift between corresponding points on certain adjacent lines between adjacent fields. Consequently by delaying the chrominance signal by an amount in time equal to one field, the necessary 180.degree. phase shift is possible with a minimum change in luminance in both the NTSC and PAL systems. However, this approach requires storing a substantial amount of video information, as the required delay is one field of 263 lines in the NTSC systems and one field of 312 lines PAL systems.
In the past, the storage of a complete field of video information has been achieved by digitizing the composite video signal and storing the results therefrom in a storage device. This storage device is commonly a semiconductor memory device. It will however be noted in this approach that as the composite video signal was initially digitized in the composite form, subsequent operations performed thereon necessarily must be performed in the digital domain. In particular, the mathematical operation of substracting the digitized form of the component composite video information from the digitized video composite signal stored in the storage means to produce a chrominance signal, as well as the implementation of the subsequent bandpass filter must be performed in the digital domain. The digital results of the signal subsequent to the bandpass filter would represent chrominance information which could thereafter be subtracted from the digitized composite video signal to produce a signal representative of chrominance. However, the R - Y and B - Y orthogonal components of the chrominance signal must still be extracted from the digitized chrominance signal. As this necessarily requires a multiplication process, implementation of same in the digital domain is particularly complex and requires extensive hardware. Efforts in this area are documented in Croll, M. G., 1980, A Digital Storage System For an Electronic Colour Rostrum Camera, IEE Conference Publication No. 191, 252-255, and Clarke, C. K. P., 1982, High Quality Decoding For PAL Inputs to Digital YUV Studieos, IEE Conference Publication No. 220, 363-366.
Consequently while it is observed that minimum changes in luminance will occur between subsequent lines on adjacent fields, implementation of this approach in the past has necessited extensive amounts of hardware to perform the required digital signal processing.